There has been a steady increase in the degree of integration in semiconductor integrated circuits in these days, and the speed of the increase is such that the degree of integration in them is increased four times in three years, as has been so said generally in this technical filed. Referring to a dynamic random access memory device as one example, those having a storage capacity of 4 M bits have been and are being produced in industrial full-scale plants in these days.
The requirements for photo-lithographic techniques which are indispensable to the production of integrated circuits are becoming severer year by year. For instance, it is said that the production of 4 M-bit DRAM needs a 0.8 .mu.m-level lithographic technique, and it is presumed that 16 M-bit or 64 M-bit DRAM having a higher degree of integration will need a 0.5 .mu.m-level or 0.3 .mu.m-level lithographic technique, respectively. Therefore, the development of resists which are acceptable in half-micron lithography is really desired.
Given the situations, exposure systems for the techniques have been and are being improved. For instance, the conventional reduction-projecting exposure method is combined with a system of a so-called phase-shift method where the phase of light is reversed, whereby the contrast of optical images to be formed may be increased extremely. According to the phase-shift method, however, positive resists form unnecessary patterns in the boundary where the phase is reversed. Therefore, high-resolution negative resists that do not form such unnecessary patterns are strongly desired.
On the other hand, as negative resists to be used in photo-lithography, well known are resists composed of a cyclized-rubber and a crosslinking agent of a bis-azide. However, the system of these resists needs organic solvents for development, by which it is swollen during development. Therefore, the resolution of images is limited to about 3 .mu.m level when the resists are used. For these reasons, the resists are unsuitable for producing integrated circuit devices having a high degree of integration. In addition, organic solvents to be used for development have various problems as they often cause environmental pollution, they are often harmful to human bodies and they are often inflammable.
Also well known are positive resists composed of a naphthoquinone diazide and an alkali-soluble novolak resin. However, since these are positive resists, it is difficult to apply them to the phase-shift method advantageous for half-micron lithography. In addition, as having a large absorption at 300 nm or less, these have another drawback that the pattern profiles to be formed using them are extremely bad when short-wavelength exposure is applied to them.
As examples that will be acceptable as high-resolution lithography, mentioned are X-ray lithography and electron beam lithography. However, the former is not satisfactory with respect to the hardwares and the resists to be used for it, while the latter cannot be utilized in industrial mass-production in view of its throughput.
Recently, a chemical amplified negative resist composed of an acid-curable resin system containing a photo acid generating agent capable of generating an acid by UV exposure (for example, refer to Japanese Patent Application Laid-Open No. 62-164045). Since the resist of this type utilizes chemical amplification, it has a high sensitivity and a high resolving power, but the stability thereof during storage is not sufficient so that it has such a problem that its sensitivity is extremely lowered when a photosensitive resist liquid containing it is stored at room temperature. Therefore, in order to use the chemical amplified resist composition for industrial mass-production of 16 M-bit and 64 M-bit semiconductor devices, it is necessary to improve the stability of the resist. In this connection, Japanese Patent Application Laid-Open No. 3-75652 mentions that the sensitivity and the stability of the resist may be improved by adding thereto a melamine resin containing hexamethoxymethylmelamine monomer in a content of 83 % by weight or more. However, the purification of hexamethoxymethylmelamine to a high degree needs distillation of the crude compound in high vacuum and high temperatures. Therefore, it is difficult to produce the intended melamine resin and the disclosed technique is not industrially practicable.
On the other hand, Japanese Patent Application Laid-Open No. 5-45879 mentions that "methylated methylolmelamine monomers are most popularly used as crosslinking agents at present, but the resolving power of the resist composition containing the monomer as the crosslinking agent is insufficient". This rather mentions that a negative resist having a high resolving power may be obtained when an oligomer of methylated methylolmelamines of a formula: ##STR1## wherein R represents an alkyl having from 1 to 10 carbon atoms, and n is an integer of from 1 to 20, is used as the crosslinking agent. However, in order to obtain the oligomer, partitioning operation by high performance liquid chromatography is necessary. Therefore, the use of the oligomer is not industrially practicable. In addition, when the oligomer is used as the crosslinking agent, the resist pattern profiles to be formed are often unfavorable, depending on the wavelength range of the light to be used for exposure.
With the improvement in the degree of integration of LSI, a large number of lithographic steps have become needed and patterning of resists in the area of the support having great difference in level has become needed. Specifically, resists capable of forming fine patterns having high aspect ratios (height of resist pattern/width of resist pattern) even though the resist film is made thick are strongly desired. Since such resists that may give good patterns even though being coated thick may be used even in ion-injecting steps and in etching steps where the selected ratio of the substrate to the resist cannot be large, they are preferred as they may be used in broad situations.
As mentioned herein above, chemical amplified negative resists comprising an acid-curable resin system and a photo acid generating agent capable of generating an acid by UV exposure have a high sensitivity and a high resolving power. However, conventional chemical amplified negative resists had such a problem, even though they might form patterns at a practical sensitivity, that the pattern profiles formed became extremely bad when the resists were coated thick so that resist patterns having high aspect ratios could not be obtained.
As mentioned herein above, resists that do not swell during development and that may be applied to phase-shift methods are indispensable for half-micron lithography. However, conventional cyclo-rubber type resists and naphthoquinone diazide-novolak type resists both do not meet the two requirements. On the other hand, conventional chemical amplified negative resists meet the indispensable requirements for half-micron lithography, but they have a serious problem with respect to the stability thereof during storage. In addition, they also have other problems with respect to the pattern profiles themselves to be formed and the aspect ratios of the formed pattern profiles, especially when the thickness of the film of the resist is made large.